If they introduced a new gay character, I don't see a problem, but involving Sam and Dean would be sure to involve huge, ugly kerfuffles. Sadly.
Oh, I have every confidence that there would be people squawking objections no matter how minor the secondary character was. I saw people vehemently objecting to the idea that Crowley might be gay (inasmuch as such things apply to demons), and he was gleefully making out with guys all over and more in-your-face with gay innuendo than any network TV character I can recall since Jack McFarland.